Outputs from an omnidirectional microphone unit and a bi-directional microphone unit can be added (combined) together to provide a unidirectional microphone (see “1-7-1 On the unidirectionality of combined microphones” co-written by Yoshinobu Yasuno, Michio Matsumoto, Hiroyuki Naono, and Mitsuhiro Katakura, Proceedings of the Meeting, The Acoustical Society of Japan, March 1982).
Examples of combinations include a combination of a stiffness controlled omnidirectional condenser unit and a resistance controlled bi-directional condenser unit and a combination of a resistance controlled omnidirectional ribbon (dynamic) unit and a mass controlled bi-directional ribbon (dynamic) unit (see a presented paper of the meeting of the Audio Engineering Society held in New York, Nov. 1, 1976, titled “A History of High-Quality Studio Microphones,” Harry F. Olson in Journal of the Audio Engineering Society, FIGS. 14 to 16, pp. 233, and related descriptions).
Omnidirectional components of a condenser microphone are stiffness controlled, and accordingly the resonance frequency of diaphragm is set to the upper limit of the sound pickup range. On the other hand, bi-directional components are resistance controlled.
In a DC biased condenser microphone, the tension on a diaphragm thereof cannot excessively be increased because the diaphragm is subject to an electrostatic attraction force from the fixed pole side. For this reason, the lower range limit for a unidirectional condenser microphone depends on the tension on the diaphragm (see Akio Mizoguchi “1-4-5 Issues on miniaturization of DC biased directional condenser microphones,” Proceedings of the Meeting, The Acoustical Society of Japan, October 1969).
Meanwhile, the type of control of a bi-directional ribbon microphone is mass control. Accordingly, the resonance frequency of the diaphragm is set to the lower limit of the sound pickup range. Since the diaphragm (metallic ribbon foil) of a ribbon microphone vibrates in a magnetic field, the electrostatic attraction force does not act on the diaphragm as would be in the case of a DC biased condenser microphone, which allows the use of a lower tension on the diaphragm.
To provide a unidirectional microphone by adding (combining) the respective outputs from omnidirectional and bi-directional microphone units together, therefore, a combination of an omnidirectional condenser microphone and a bi-directional ribbon microphone is desirable in that there are less restrictions in terms of frequency.
However, the output level of a ribbon microphone unit is lower than that of a condenser microphone unit. The problem, therefore, is that an output from a condenser microphone unit and an output from a ribbon microphone unit cannot simply be added together in an attempt to provide a unidirectional microphone by combining the respective sound signal outputs from the units.
Accordingly, an object of the invention is to provide a unidirectional microphone by adding an output from an omnidirectional condenser microphone unit and an output from a bi-directional ribbon microphone unit together.